The signs of a behavioral health problem in children vary by age and type of issue. There are two warning signs tend to cross over into all categories and signal that you should get help from a health professional experienced in childrens' psychology as soon as possible:

• Extremes or peculiarity of behavior for the age and gender of the child, such as being significantly more hyper, aggressive, or withdrawn.
• Sudden, hard-to-explain negative changes in behavior, such as a steep drop in grades.

Here are some of the signs of mental illness during different age ranges:

Preschool/early elementary school years

• Behavior problems in preschool or day care.
• Hyperactivity way beyond what the other children are doing.
• Trouble sleeping.
• Persistent nightmares.
• Excessive fear, worrying, or crying.
• Extreme disobedience or aggression. Because it's often within a child's nature to disobey or intrude on a playmate's space, an excessive degree of this behavior is what should concern you, such as deliberate destructiveness or hurting peers or animals.
• Lots of temper tantrums all the time.
• Persistent difficulty separating from a parent. Although many children experience separation anxiety at times, there could be a problem if this goes on for months.

Elementary/middle school years

• Inability to maintain friendship/friends dropping off
• Excessive fears and worries
• Extreme hyperactivity
• Sudden decrease in school performance
• Loss of interest in friends or favorite activities
• Loss of appetite
• Sudden changes in weight
• Excessive worry about weight gain
• Sudden changes in sleep habits
• Visible prolonged sadness
• Substance use or abuse
• Seeing or hearing things that are not there

Middle/high school years

• Destructive behavior, such as damaging property or setting fires.
• Constantly threatening to run away or actually running away, which can be a precursor to self-harm.
• Withdrawal from family and friends.
• Comments or writings that suggest a desire to harm themselves or others.